Obtain source address from route - c++

Question

In this users example a route is obtained by using the command line utility ip in linux. Example output:
$ ip route get 4.2.2.1
4.2.2.1 via 192.168.0.1 dev eth0 src 192.168.0.121
cache
$
Let's refer to the addresses in the following manner:
4.2.2.1 as address A (destination)
192.168.0.1 as address B (gateway)
192.168.0.121 as address C (source)
In my case I'm interested in C - and I'm trying to figure out how I might be able to obtain the same piece of information in my c++ program. Specifically
Given address A, find address C
do not want to use system or anything that will somehow run a shell command
Using boost is allowed, and preferred
Any recommendations? Thanks

Answer 1

There you go:
#include <iostream>
#include "boost/asio/io_service.hpp"
#include "boost/asio/ip/address.hpp"
#include "boost/asio/ip/udp.hpp"
boost::asio::ip::address source_address(
const boost::asio::ip::address& ip_address) {
using boost::asio::ip::udp;
boost::asio::io_service service;
udp::socket socket(service);
udp::endpoint endpoint(ip_address, 0);
socket.connect(endpoint);
return socket.local_endpoint().address();
}
// Usage example:
int main() {
auto destination_address = boost::asio::ip::address::from_string("8.8.8.8");
std::cout << "Source ip address: "
<< source_address(destination_address).to_string()
<< '\n';
}

Answer 2

mash's answer is almost right but fails on iOS. The line udp::endpoint endpoint(ip_address, 0); needs to have a non-zero port or you'll get the error "Can't assign requested address" since 0 is not a valid port number. I don't think it matters what the port is (as long as it's a valid non-zero port number) so I would recommend using 3478 which is the standard UDP STUN port.
Corrected code:
#include <iostream>
#include "boost/asio/io_service.hpp"
#include "boost/asio/ip/address.hpp"
#include "boost/asio/ip/udp.hpp"
boost::asio::ip::address source_address(
const boost::asio::ip::address& ip_address) {
using boost::asio::ip::udp;
boost::asio::io_service service;
udp::socket socket(service);
udp::endpoint endpoint(ip_address, 3478);
socket.connect(endpoint);
return socket.local_endpoint().address();
}
// Usage example:
int main() {
auto destination_address = boost::asio::ip::address::from_string("8.8.8.8");
std::cout << "Source ip address: "
<< source_address(destination_address).to_string()
<< '\n';
}